It is well-known in the electronic materials art that the impurity density in single crystal ingots of materials may be reduced by establishing a molten zone in such material and then moving the zone along the material one or more times at controlled rates and temperatures. Materials for which this process is well established include silicon, germanium, Group III-V compounds such as gallium arsenide and Group II-VI compounds such as cadmium telluride. During this zone refining process, the molten zone produces continuous melting and crystallization of the ingot, as is also well-known. This process has the effect of increasing the purity of the ingot by, among other things, moving the randomly distributed impurities in the ingot toward one end of the ingot. As a result of the movement of the liquid-solid interface along the length of an ingot, these impurities are swept toward one end of the ingot where they build up in quantity with time during a number of successive zone refining passes and from where they can subsequently be removed.
Examples of references relating to zone refining processes and apparatus include U.S. Pat. Nos. 2,773,923 and 3,909,246 and Zone Melting, (2nd Ed.), W. G. Pfann, John Wiley & Sons, New York (1966). Both patents disclose a trough for containing a charge of material to be zone refined. While these patents are useful for zone refining materials for which the final cross sectional configuration is unimportant, they are not suitable for zone refining materials of substantially round cross section, such as cylindrical ingots. Pfann describes the problems of matter transport and container cracking (pp. 47-52).
There is a need for high purity alkali halide crystals of cylindrical configuration (e.g., in the manufacture of fibers), which must be further purified following preparation. Attempts to zone refine such materials employing conventional open containers, such as those disclosed above, result in substantial loss of the round cross section; see the Pfann reference, p. 73. On the other hand, attempts to zone refine such cylindrical materials in completely closed tubular containers result in fracture of the containers. Such fracture occurs for those materials which evidence a volume change upon phase change (e.g., solid to liquid).